Portable data carriers, such as chip cards or transponders, are hitherto dependent mainly on an external power supply. For supplying power to a batteryless transponder, a high-frequency magnetic alternating field of an RFID reading device (typical frequencies: 125 kHz, 13.56 MHz) is normally used. For this purpose, an alternating voltage induced by the magnetic alternating field in the antenna coil of the transponder is rectified and supplied to the transponder as supply voltage. For voltage regulation in the transponder, so-called shunt regulators are used. Surplus energy is thereby conducted into a shunt resistor and converted into heat.
To permit operation of a portable data carrier independently of an external power supply, portable data carriers can be provided with a battery. In particular for transponders or contactless chip cards, it should also be possible to charge the battery by a charging apparatus contactlessly, that is, not by a corresponding electroconductive contacting.
For contactless charging apparatuses, it is known to use the energy transferred through a close magnetic coupling (coils with a ferrite core) for charging an accumulator. Such an arrangement is for example used frequently in electrical tooth cleaning apparatuses to permit the handset to be supplied with power for charging the accumulator without an external galvanic contact.
The prior art comprises methods for data transfer at close range, such as NFC (near field communication). NFC is a transfer method by means of magnetic fields in the frequency domain of 13.56 MHz. The typical range of NFC devices is about 20 cm. The methods used for data transfer are very similar to those of contactless chip cards. NFC devices also therefore are able to communicate with contactless chip cards, or to simulate a contactless chip card (producing a load modulation). The NFC specification provides for not only a “peer-to-peer” mode, in which both communication partners transmit alternately, but also a mode in which a first NFC device changes to a permanent transmit mode (“being reader”) and a second NFC device simulates a contactless chip card (“being card”) to be able to communicate with the first device by load modulation in this way. For the field strengths of a carrier signal and of the corresponding sidebands of an NFC device, limiting values are defined which the radiated energy of the NFC device must not exceed. The field strength of a field usable for energy transfer is thus also limited.
To obtain an improved inductive coupling between a chip card with an antenna coil and a charging apparatus, DE 199 40 561 C1 uses as a charging apparatus a pin with a ferrite core and a transmitting coil. For energy transfer, the pin is inserted into a hole which is disposed in the chip card in the area of the antenna coil.
The U.S. Pat. No. 6,275,681 B1 shows a method for contactless charging of the power supply unit of a chip card by means of an electrostatic charging apparatus of a chip card reading device. The chip card comprises a control unit which controls the charging of the power supply unit in dependence on its state of charge, provided a sufficient voltage supply is available. The information on whether charging has been carried out completely or incompletely can be transferred from the chip card to the reading device and displayed to a user of the chip card.
The time duration of a process is particularly critical with contactless processes in comparison with contact-type processes. A user will for example consciously not hold his chip card in the area of a reading device for an indefinitely long time, or with increased probability unconsciously remove it from the communication area of the reading device.